Carbon-Based Fluorescent Tracers as Oil Reservoir Nano-Agents

ABSTRACT

The present invention relates to carbon-based fluorescent nano-agent tracers for analysis of oil reservoirs. The carbon-based fluorescent nano-agents may be used in the analysis of the porosity of a formation. The nanoagents are suitable for injection into a petroleum reservoir and may be recovered from the reservoir for the determination of hydrocarbon flow rates and retention times.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 14/627,404, filed Feb. 20, 2015, which is a is adivisional of and claims priority to U.S. patent application Ser. No.13/469,459, filed on May 11, 2012, which claims priority to U.S.Provisional Patent Application No. 61/486,090, filed on May 13, 2011,the disclosures each of which are incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates to carbon-based fluorescent nano-agenttracers for analysis of oil reservoirs, and methods of making and usingsame.

BACKGROUND OF THE INVENTION

Crude oil is the world's main source of hydrocarbons that are used asfuel and petrochemical feedstock. One overriding problem in exploringfor hydrocarbons in the #5285223.2 subsurface is related to the probingand characterization of an environment that cannot be seen. Similarly,after a hydrocarbon deposit has been discovered and is ready to bedeveloped and exploited, many assumptions must be made by reservoirgeologists and reservoir engineers in the modeling of a large volume ofrock which cannot be seen.

Subsurface reservoir data is traditionally acquired by lowering probesinto boreholes for purposes of sampling and/or probing the reservoir andfrom images obtained through seismography. In the first instance, thedata is handicapped by the insufficiency of the resulting data, byvirtue of being sourced from a single 6-inch hole, thus giving a verynarrow of a view of the reservoir as a whole. Interpreted seismicvolumes, on the other hand, typically give too broad of a view due totheir imaging quality and resolution inadequacies. Even combining thetwo data types, does not enable for the mapping of exact highpermeability pathways.

The integration of available geological, geophysical, petrophysicalengineering, and drilling data makes interesting inroads into thedetection, mapping and predictive modeling of high permeabilitypathways. The final uncertainty of integrated models, however, can onlybe marginally better than the average uncertainty inherent in each thevarious methods used. Mix and integrate the data as much as one may, thebroad brush strokes on reservoir map deliverables will remain just that:broad brush. For example, a 0.5 mm scribble drawn on a 1:200,000 scalemap to represent a fracture in the subsurface, is akin to depicting afracture with a 200 meter aperture because of the width of the scribblerelative to the scale of the map. Nor does the scribble reveal theprecise path that the fluids are likely to take.

Additionally, as oil fields mature, it is expected that fluid injectionfor pressure support (i.e., secondary enhanced oil recovery) willincreasingly tend to erratically invade and irregularly sweep theresidual oil leg. Concerns have led to needs to identify, detect and mappathways that may lead injected fluids prematurely updip alongencroachment fingers. More often than not, the encroachment materializesfaster than even the worst case expectations, and commonly in quiteunpredictable directions. Moreover, premature encroachment is commonlytortuous and will change direction in 3D volume. This type of tortuosityrenders high permeability pathway prediction nearly impossible tosatisfactorily pin down. In spite of an arsenal of cutting-edgetechnologies that can be thrown at such problems, high permeabilitypathway prediction capability continues to suffer from high levels ofuncertainty.

Even with current technology, it is impossible to work out and predictan exact pathway that fluid fingering will take as it invades deep intoan oil leg, much less where it will go next. Engineering data (e.g.water arrival data, including, water arrival detected in an oilproducing well, flowmeter data, test pH build-up, pressure data, andproduction/injection data), although mostly acquired at the borehole,are typically correlated aerially. The resultant maps provide anindirect, unreliable and a crude way of trying to depict the geology ofa reservoir. The resultant maps are interpretive, and reservoirengineers are the first to dissociate them from being accuratereflections of specific geologic features. Moreover, the map resolutionsare too broad to even remotely represent most geological features thatwould commonly be associated with high permeability pathways.

Other interwell methods to map permeability pathways are, likewise,handicapped by resolution problems. Geophysical technologies rooted ininterpreting 3D, 4D, shear wave, or multi-component volumes; even whenutilizing ever-developing clarity and resolution enhancing softwarepackages, still only render a generalized mapping of a minisculesampling of some faults in the general area where they may or may not belocated.

In carbonate rock formations, fractures having apertures that aremeasured in millimeters, or geobodies only centimeters across, canprovide the necessary plumbing to take injected fluid past matrixed oil.To further illustrate this, a 3 cm wide fracture with no displacementmay, under pressure, move fluids at several Darcies. These dimensionscannot be seen by current interpretive geophysical devices.Subsequently, the fault lines drawn on reservoir structure maps cannotbe considered more than broad arrows pointing out a general direction;and not a depiction of actual permeability pathways. Furthermore,geophysically-interpreted data must be augmented by a solidunderstanding of the regional stress-strain regimes in order to filterout fracture swarms which may not be contributing to premature fluidbreakthroughs.

Dyes and radioactive chemical tracers that can be introduced withinjected fluids can be helpful locally, but generally do not reveal theactual pathway that is taken by the host fluid from the entry well tothe detection well. Borehole detection methods are the most exact, butare also plagued with major shortcomings, such as for mapping purposes,wellsite data must be extrapolated and transformed into interwellinformation. Extrapolation in itself creates many problems. Somedisadvantages associated with molecular (chemical) tracers includediffusivity and adsorption. Molecular tracers, due to their small size,tend to diffuse into all of the small pores of a matrix (as comparedwith larger tracers), and thus take longer periods of time to travelbetween the injection well and the production well. Additionally,adsorption of the molecular tracers can also be a factor, requiring theinjection of much larger quantities of the chemical tracers than isdesired.

The slightest shifts in water depth, measured in decimeters, can createworlds of difference in sediment deposition. Rock minerals, especiallycarbonates, are in continuous “life long” effective diagenesis from theinstant of deposition. Carbonate porosity is dictated by deposition andunceasingly altered by diagenesis.

Geostatistical distribution of attributes, including fractures detectedon borehole image logs, at the wellbore, is only statistical, andnatural geological landscapes are too variable to respond comfortably tothe smooth, clean logic of mathematics. There are no two features incarbonate rocks that are the same.

Nanotechnology brings new and different capabilities into upstreamexploration and production. The industry desires strong, stable,friction resistant, and corrosion combatant materials in virtually allof its operations and processes (e.g., pipes, casings, drill strings,production tubings, and drill bits). These requirements are morefavorably addressed with a bottom-up approach for material design andfabrication, and by employing nanofabricated particles for use indrilling, completion, stimulation, and injection fluids. Use of thesematerials allows faster drilling, prevents near wellbore damage, minehydraulic fractures, plug water thief zones, reduce waterfloodfingering, encourage or enhance oil production, and prevent waterbreakthroughs. It is hoped that the use of certain nano-based agents maysoon lead to the development and deployment of sensing and interventiondevices that can help delineate the waterflood front, identify bypassedoil, and map super permeability zones in-situ in the underground. Thecapabilities become limitless with the possibility of havingfunctionalized molecular agents that “illuminate” the reservoir andpossibly intervene to rectify adverse transport conditions in themedium.

As worldwide petroleum reserves decrease and their recovery becomesincreasingly difficult, methods for locating and mapping petroleumreservoirs becomes more and more critical. Due to the high pressures andtemperatures that are encountered in subsurface formations, materialsthat are able to withstand these conditions are needed. Thus, there is aneed for the development of new materials for use with the mapping ofpetroleum reservoirs.

SUMMARY OF THE INVENTION

The present invention relates to carbon-based fluorescent nano-agenttracers for analysis of oil reservoirs, and methods of making and usingsame. In some embodiments, a fluorescent nanoagent for use in asubsurface petroleum reservoir is provided, the nanoagent including acarbon-based nanoparticle core, said nanoparticle core having an averagediameter of less than about 100 nm; wherein said nanoagent includes aplurality of fluorescent functional groups appended to the surfacethereof. In further embodiments, the carbon-based nanoparticle core hasan average diameter of less than about 75 nm. In still furtherembodiments, the fluorescent nanoagent is detectable at a concentrationof about 0.01 ppm. In certain embodiments, the functional groupsappended the surface of the fluorescent nanoagent can be excited at awavelength between about 400 nm and 500 nm.

In some embodiments, the functional group appended to the surface of thenanoparticle core is an amino alcohol. In further embodiments, thefunctional group appended to the surface of the nanoparticle core isselected from methanolamine, ethanolamine, and propanolamine. In certainembodiments, the functional group appended to the surface of thenanoparticle core is ethanolamine. In further embodiments, thefunctional group appended to the surface of the nanoparticle core isappended with an amide linkage. In still further embodiments, thefluorescent nanoagent is produced from a solution comprising a sugar, anamino alcohol and deionized water reacted under conditions capable ofsynthesizing the fluorescent nanoagent.

In some embodiments, the sugar is selected from the group consisting ofglucose and fructose. In certain embodiments, the amino alcohol isselected from the group consisting of methanolamine, ethanolamine, andpropanolamine. In further embodiments, the functional group is anorganic functional group. In still further embodiments, the organicfunctional group is present in an amount of between about 50% and 90% byweight. In certain embodiments, the fluorescent nanoagent is stable at atemperature of about 100° C. to about 200° C. In some embodiments, thefluorescent nanoagent is stable at a salinity concentration of about75,000 ppm to about 120,000 ppm.

In certain embodiments, a method for the preparation of a fluorescentnanoagent for use in a subsurface petroleum reservoir is provided. Themethod includes the steps of heating an aqueous mixture of citric acidand ethanolamine at a temperature of about 70° C. to remove the majorityof the water and produce a viscous solution; heating the viscoussolution at a temperature of at least about 200° C. for at least about 2hr; and collecting the resulting black particles products, saidparticles having an average diameter of about 10 nm, and wherein saidparticles include a fluorescent group appended to the surface thereof.

In further embodiments, a method for the preparation of a fluorescentnanoagent for use in a subsurface petroleum reservoir is provided. Themethod includes the steps of heating an aqueous solution of glucose in ahigh pressure autoclave at a temperature of at least about 150° C. forat least about 4 hrs; adding ethanolamine to the solution and refluxingthe resulting mixture for a period of at least about 10 hrs; collectinga solid product comprising carbon-based nanoparticles having fluorescentfunctional groups attached to the surface thereof.

In another aspect, a method for analyzing a subsurface petroleumformation is provided. The method includes the steps of injecting afluid comprising a plurality of nanoagents prepared according to thedescription above into an injection well, said injection well beingfluidly connected to the subsurface petroleum formation; recovering thefluid injected into the injection well at a production well, saidproduction well being fluidly connected to said subsurface petroleumformation; and analyzing the recovered fluid for the presence of thenanoagents present therein.

In some embodiments, the fluorescent nanoagent is produced from asolution comprising a sugar, an amino alcohol and deionized waterreacted under conditions capable of synthesizing the fluorescentnanoagent. In further embodiments, the sugar is selected from the groupconsisting of glucose and fructose. In still further embodiments, theamino alcohol is selected from the group consisting of methanolamine,ethanolamine, and propanolamine. In some embodiments, the functionalgroup is an organic functional group. In certain embodiments, theorganic functional group is present in an amount of between about 50%and 90% by weight. In some embodiments, the fluorescent nanoagent isstable at a temperature of about 100° C. to about 200° C. In certainembodiments, the fluorescent nanoagent is stable at a salinityconcentration of about 75,000 ppm to about 120,000 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of theinvention, as well as others which will become apparent, are attained,and can be understood in more detail, more particular description of theinvention briefly summarized above may be had by reference to theembodiments thereof which are illustrated in the accompanying drawingsthat form a part of this specification. It is to be noted, however, thatthe drawings illustrate only several embodiments of the invention andare therefore not to be considered limiting of its scope as theinvention may admit to other equally effective embodiments. The presenttechnology will be better understood on reading the following detaileddescription of non-limiting embodiments thereof, and on examining theaccompanying drawings, in which:

FIG. 1 provides an X-ray diffraction (XRD) pattern for one embodiment ofthe invention;

FIG. 2 provides a comparison of stability of one embodiment of theinvention as compared with a control sample;

FIG. 3 provides a comparison of washed and unwashed samples of oneembodiment of the present invention using optical and confocalmicroscopy;

FIG. 4 provides a Fourier transform infrared spectroscopy (FTIR)spectrum of one embodiment of the present invention;

FIG. 5 provides a transmission electron microscopy (TEM) image of oneembodiment of the invention;

FIG. 6 provides a measure of the zeta-potential of one embodiment of theinvention;

FIG. 7 provides the fluorescence measure of one embodiment of theinvention;

FIG. 8 provides a transmission electron microscopy image of oneembodiment of the invention;

FIG. 9a provides a measure of average particle size of one embodiment ofthe invention;

FIG. 9b provides a measure of the zeta-potential of one embodiment ofthe invention;

FIG. 10 provides the fluorescence measure of one embodiment of theinvention;

FIG. 11a provides a measure of average particle size of one embodimentof the invention;

FIG. 11b provides a measure of the zeta-potential of one embodiment ofthe invention;

FIG. 12 provides coreflood test results for one embodiment of thepresent invention; and

FIG. 13 provides a thermogravimetric analysis (TGA) of one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specificdetails for purposes of illustration, it is understood that one ofordinary skill in the art will appreciate that many examples, variationsand alterations to the following details are within the scope and spiritof the invention. Accordingly, the exemplary embodiments of theinvention described herein and provided in the appended figures are setforth without any loss of generality, and without imposing limitations,relating to the claimed invention.

In one aspect, the present invention is directed carbon-basedfluorescent nanoparticle tracers (hereinafter referred to as “A-Dots”and “A-NPs”) as either stand-alone particles or building blocks for usein analyzing petroleum reservoirs. The nanoparticle tracers havesufficient long term stability in both high temperature and highsalinity environments, allowing the tracers to survive transit throughthe subsurface between wells. The nanoparticle tracers can be introducedinto the reservoir through an injector well, where they flow through thereservoir. They can be detected, sampled, examined and tested reliablywhen the nanoparticles are recovered back at the surface from a producerwell. In certain embodiments, the nanoparticles can include nanosizedfullerene or carbon nanotubes. In alternate embodiments, thenanoparticles are not fullerene or carbon nanotube materials. In certainembodiments, the nanoparticles can include materials having a strongfluorescence signature.

In certain embodiments, the nanoparticle tracers are stable attemperatures up to about 100° C. or greater, alternatively 200° C. orgreater, and at salinity concentrations of about 75,000 ppm,alternatively about 120,000 ppm or greater in the presence of monovalentand divalent ions, such as Ca⁺², Mg⁺², and SO₄ ⁻². The nanoparticletracers are also stable when in contact with the carbonate reservoirrock environment. The nanoparticle tracers of the present invention arethe first reservoir nanoagents in the industry that are known to bestable under these reservoir conditions.

Carbon nanoparticles represent a unique class of nanomaterials that aretypically synthesized through a hydrothermal treatment process.Analogous to their well-known cousins, fullerenes and carbon nanotubes,carbon nanoparticles can exhibit interesting optical (and electrical)properties that may be exploited in applications such asoptoelectronics, chemical sensing, biological labeling, and in thiscase, the upstream oil and gas. For use herein, the carbon nanoparticlescan have an average diameter of less than about 100 nm, alternativelyless than about 75 nm, or alternatively less than about 50 nm. Incertain embodiments, the carbon nanoparticles can have an averagediameter of between about 10 nm and 50 nm; alternatively between about10 nm and 30 nm. In certain embodiments the carbon nanoparticles have adiameter of between about 5 nm and 75 nm; alternatively between about 5nm and 50 nm. An X-ray diffraction (XRD) pattern of the A-Dots is shownin FIG. 1. The pattern shows the characteristics of a poorly crystallinegraphite structure at 2θ=21.8°, with (002) interplanar spacing of 4.2 Å.

The A-Dots and A-NPs described in certain embodiments herein are passivereservoir carbon nanoparticle-based nanoagents that target reservoirsfor in-situ sensing and intervention. Exemplary intervention activitiescan include acting to rectify unfavorable oil sweep and recoveryconditions existing in a reservoir, such as plugging super-permeablezones for enhanced conformance during waterfloods or for deliveringchemicals to targets deep within a reservoir to alter wettability,reduce interfacial tensions, and/or enhance oil recovery. It isunderstood that the carbon nanoagents described herein can be used forother intervention activities beyond the exemplary activities listedabove. In-situ sensing and intervention is a highly appealing conceptfor analyzing petroleum formations. In view of the severe reservoirconditions of temperatures of up greater than about 100° C. and salinityconcentrations of up to about 120,000 ppm or greater, a fundamentalchallenge in utilizing nanoparticle tracers to determine formationproperties and conditions is the stability of the nanoagents in thesubsurface medium.

As the A-Dots and A-NPs are the first nanoparticles in the industry(currently in passive form) that have proven stability in subsurfaceconditions, these nanoparticles may, in certain embodiments, alsoprovide a template or design that become basis for the preparation offunctional active and/or reactive nanoagents. One possible use is withthe nanoagents disclosed in U.S. Ser. No. 12/722,357, filed on Mar. 11,2010, which claims priority to U.S. Prov. Pat. App. Ser. No. 61/159,943,filed on Mar. 13, 2009.

As noted previously, one important unique feature of both the A-Dots andthe A-NPs is their in-reservoir stability. More specifically, both theA-Dots and A-NPs have been found to be stable at temperatures of betweenabout 100° C. and 150° C., and greater, and also in a brine solutionthat includes the following compounds at the following concentrations:NaCl (128.9 g/L), CaCl₂.2H₂O (109.16 g/L), MgCl₂.6H₂O (35.66 g/L), BaCl₂(0.02 g/L), Na₂SO₄ (0.16 g/L) and NaHCO₃ (0.48 g/L), totaling aconcentration of about 120,000 ppm of total dissolved solids. Generally,the A-Dots and A-NPs have been found to be stable in water,alternatively in brine solutions having a total dissolved solidsconcentration of between 100 ppm and 25,000 ppm, alternatively in brinesolutions having a total dissolved solids concentration of between25,000 ppm and 50,000 ppm, alternatively in brine solutions having atotal dissolved solids concentration of between 50,000 ppm and 100,000ppm, alternatively in brine solutions having a total dissolved solidsconcentration of greater than 100,000 ppm. Stability has alsodemonstrated in connate water having a concentration of 220,000 ppm TDS.

To gauge the A-Dots adsorption potential and affinity to carbonate rockformations, ample amounts of either crushed limestone (coarse grain) orCaCO₃ powder were used. Small amounts of oil from the Arab-D reservoirwere also added to gauge the hydrophilicity of the nanoparticles. Thesamples were shaken and heated to a temperature of up to about 150° C.for up to 12 days. Fluorescence spectra were measured before and aftereach test. According the Arrhenius law, accelerated testing at about150° C. increased the exposure time 32-fold for the target reservoirtemperature of about 100° C.

The results showed A-Dots to be stable at high temperatures and highsalinity conditions for very long durations (i.e., for up to about 12days at 150° C.). The A-Dots also maintained strong fluorescence afterexposure to high temperatures and high salinity conditions with nodiscernible particle aggregation (as shown by TEM). The A-Dots alsoremained colloidally stable, even after centrifugation, with no visiblesedimentation or loss of fluorescence. Referring to FIG. 2, theimportance of proper surface functionalization on the nanoparticlestability is shown. As used herein, proper functionalization refers tothe introduction of functional groups to the surface of thenanoparticles that allows the particles to remain dispersed andsuspended, and without having an affinity to stick or bind to carbonatepresent in the reservoir environment. Suitable functional groupsgenerally include primary amines and primary amino alcohols. One suchexemplary amino alcohol is ethanolamine. In certain embodiments,methanolamine and propanolamine can also be used. In certainembodiments, secondary amines or alcohols can be used. In otherembodiments, the functional groups can be selected based upon theirability to exhibit high fluorescent yields, and may be selected basedupon the reactivity of the fluorescent functional groups with aformation. In certain embodiments, the organic functional groups arepresent in an amount of between about 50% and 90% by weight,alternatively between about 60% and 80% by weight, alternatively betweenabout 70% and 80% by weight, alternatively between about 65% and 75% byweight.

FIG. 2 shows a comparison of the stability of the A-Dots nanoparticlesystem to a typical nanoparticles solution (for example, a 100 ppm or100 ppm solution of non-functionalized nanoparticles) following thermaltreatment for about 5 days at a temperature of about 120° C. in highsalinity brine (i.e., at least about 120,000 ppm TDS). Tests wereconducted under similar conditions for the two nanoparticle solutions.As shown in FIG. 2, ethanolamine functionalized A-Dots at aconcentration of about 1000 ppm remained stable over the course of thetest, as depicted in FIGS. 2d and 2e . In contrast, as shown in FIGS. 2aand 2b , greater amounts of flocculation and sedimentation occur for asolution having a 1000 ppm concentration of nanoparticles that have beenfunctionalized with JEFFAMINE®. FIG. 2c shows sedimentation following asecond cycle of heat treatment on the supernatant from the sample inFIG. 2b . Colloidal stability and hydrophilicity testing in the presenceof hot brine, CaCO₃ fines, and Arab-D oil for the A-Dots is presented inFIG. 2 f.

FIG. 3 shows carbonate crystals under both optical and confocalmicroscopy for the same crystal following exposure to A-Dots solution,providing the affinity of the A-Dots to the carbonate crystals. Theunwashed crystals are clearly visible under confocal microscopy due tothe residue of the fluorescent A-Dots; whereas, the washed crystals arenot. This demonstrates that the A-Dots are not stuck or bonded to thecarbonate crystals, but are instead merely sitting on them, thusindicating a likelihood that the A-Dots will travel through carbonateformations.

A-Dot fluorescence is high and, in certain embodiments, can bedetectable at levels below about 5 ppm, alternatively at levels belowabout 1 ppm, alternatively at levels below about 0.5 ppm, alternativelyat levels below about 0.1 ppm, or at levels below about 0.05 ppm. Incertain embodiments, it is possible to detect the presence of A-Dots atconcentrations as low as 1 ppb (i.e., 0.001 ppm). Fluorescence quantumyield (i.e., photons emitted/photons absorbed) can be about 10%, and isvirtually independent of the excitation wavelength, which can be in therange of between about 400 to 500 nm. Emission is preferably monitoredat a wavelength of about 460 nm, although it is possible to monitor theemission at other wavelengths as well. In certain embodiments, uponexcitation, the A-Dot fluorescence can be detected at a wavelength ofbetween about 450 nm to about 475 nm.

In certain embodiments, the intensity of the fluorescence is notaffected by the presence of CaCO₃, demonstrating that use in carbonateformations does not reduce the fluorescence of the tracers. Withoutwishing to be bound by any single theory, fluorescence is believed todepend upon the formation of amide linkages on the surface of the carbonnanoparticles, which may be responsible for the majority of thefluorescence. This is shown in FIG. 4, which shows an FTIR spectrum(bottom trace) of one embodiment of the A-Dots, wherein peaks at about1690 cm-1 and 1550 cm-1 are characteristic of the presence of amidelinkages. The top trace is the FTIR spectra of citric acid, and themiddle trace is the FTIR spectra of ethanolamine.

A-Dot solubility was tested in deionized water, seawater (having aconcentration of about 50,000 ppm TDS), high salinity brine (having aconcentration of about 120,000 ppm TDS), and super-high salinity brine(having a concentration of about 230,000 ppm TDS) at temperatures ofbetween about 100° C. and 150° C.

In general, the A-Dots can be synthesized in a simple, one-pot reaction.Carbon nanoparticles are produced hydrothermally, followed by surfacefunctionalization. Accordingly, the process is very amenable forscale-up to the kilogram level in a non-industrial research labenvironment. The synthesis is also very economical, with a current costof less than about $10.00/kg. As the A-Dots can be detected atconcentrations that are below 100 ppb concentrations, the labelinginjection water with the A-Dots to a level of about 10 ppm (which is 100times greater than the minimum detection limit of the A-Dots) can bedone at a current cost of about $20.00 per thousand barrels of injectionwater, which is several orders of magnitude less expensive thanconventional molecular tracers that are currently being used.

In contrast, the larger A-NPs carbon nanoparticles can be prepared in atwo-step process. The synthesis of the A-Dots and A-NPs proceeds asfollows.

Examples

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventor to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

Chemicals:

Ethanolamine (99%), citric acid monohydrate (99.5%), glucose(anhydrous), NaCl (99%), MgCl₂.6H₂O, CaCl₂.2H₂O (99%), BaCl₂ (99.9%),Na₂SO₄ (99%) and NaHCO₃ (99.7%) were purchased from Sigma-Aldrich andwere used as received without further purification.

Characterization:

TEM images were obtained using a Tecnai FEI T12 transmission electronmicroscope operated at 120 kV. Samples were prepared by placing a smalldrop of an aqueous suspension on a carbon-coated copper grid. Excesssolution was wicked off using a blotting paper. The grid was then driedin air prior to imaging. Dynamic Light Scattering and zeta potentialmeasurements were carried out using a Malvern Zetasizer (MalvernInstruments, Nanoseries). Each sample was measured three times and theaverage value was used. Photoluminescence spectra were obtained using aMolecular Device Spectra Max M2^(e) spectrophotometer equipped with axenon flash lamp. The excitation wavelength was 385 nm.

Example 1

Synthesis of A-Dots (Carbon-dots). In a typical process, 4.2 g citricacid monohydrate and 3.66 g ethanolamine were dissolved separately in 10mL of deionized water and mixed using a magnetic stirrer. The reactionmixture was then heated to about 70° C. on a hot plate under constantstirring to evaporate the majority of the water from the reactionmixture. When the reaction mixture became syrupy, the magnetic bar wasremoved and the reaction mixture was placed in a furnace and heated ofat least about 200° C. in air at a rate of 10° C./min for at least about3 hrs. The resulting black product was allowed to cool to roomtemperature and used without any further purification. The products arehighly soluble/dispersible in water.

In certain embodiments, the synthesis of the A-Dots leads to uniformlysized hybrid nanoparticles (hairy) that are readily dispersible in waterand organic solvents, depending upon the length and type of ligands thatare attached to the carbon core.

The amount of organic compound present on the surface as determined bythermogravimetric analysis (TGA) and shown in FIG. 13 is approximately75% by weight. From left to right, FIG. 13 shows the TGA of ethanolamine, citric acid, 3:1 A-Dots and 2:1 A-Dots.

Example 2

Scale up Synthesis of A-Dots (Carbon-dots). For the scale up synthesisof A-Dots, 378 g of citric acid monohydrate and 330 g of ethanolaminewere separately added to two large beakers and combined with enoughdeionized water was added to each to bring the volume to 900 mL. Aftercomplete dissolution, the ethanolamine solution was added to the citricacid solution under constant magnetic stirring. The mixture was stirredand heated at 70° C. on a hot plate to evaporate most of the water.After the volume was reduced to about 650 mL, the mixture wastransferred to 1 L glass bottles and heated at 200° C. in air at a rateof 10° C./min for 8 hrs.

Example 3

Characterization of A-Dots. The carbon-dots were synthesized via aone-pot reaction using the citrate salt as a precursor. When citric acidand ethanolamine are mixed, the corresponding ammonium carboxylate saltis formed. During pyrolysis, the citrate part of this precursor moleculeprovides the source of carbon for the core with the remaining ammoniumgroups attached to the surface. The reaction leads to uniform sizehybrid (hairy) nanoparticles in high yield that are readily dispersiblein water or organic solvents, depending on the type and length ofammonium hairs attached to the carbon core. Referring to FIG. 5, TEMimages of the carbon-dots prepared from an aqueous solution show uniformparticles having an average diameter of less than about 10 nm. Referringnow to FIG. 6, the resulting nanoparticles are negatively charged havinga zeta potential of about −26 mV. Without wishing to be bound by anysingle theory, the negative charge and zeta potential are possibly dueto the presence of carboxylate groups on the surface of thenanoparticles.

Referring to FIG. 7, the carbon-dots are highly fluorescent and can bedetected at concentrations below 2 ppm, alternatively at concentrationsless than about 1 ppm, alternatively at concentrations of about 0.1 ppm,alternatively at concentrations of about 0.01 ppm, or alternatively atconcentrations of about 0.001 ppm. FIG. 7 shows that above certainconcentrations of the carbon A-dots, the fluorescence intensity becomessaturated and starts decreasing due to self-quenching. The fluorescencequantum yield of the A-Dots is approximately 10% and is virtuallyindependent of the excitation wavelength within the range of 400-500 nm,thus making the A-Dots excellent emitters that are detectable atrelatively low concentrations.

Example 4

A-NP (Carbon Nanoparticles) Synthesis. Carbon nanoparticles weresynthesized by hydrothermal treatment. In a typical reaction, 0.61 g ofglucose was dissolved in 17 mL of deionized water, although othersugars, such as fructose, can be used. The resulting solution was thentransferred to a high pressure stainless steel autoclave fitted with aglass liner and sealed. The reactor was heated to 160° C. in an oven for4 hrs at a heating rate of 10° C./min. After cooling a dark brownsolution was obtained and used without further purification. For surfacefunctionalization, approximately 20 mL of the solution was transferredto a round bottom flask fitted with a condenser and diluted to 80 mLwith deionized water. Approximately 4 mL of an aqueous solutioncontaining about 0.8 mL of ethanolamine was added to the flask and themixture was refluxed for approximately 12 hrs. After cooling to roomtemperature, the resulting products were purified by dialysis in waterusing a cellulose membrane (having a molecular weight cut-off of 7000).

Example 5

A-NP Characterization. Carbon A-NP nanoparticles were prepared by thetwo-step process described above. In general, as determined by TEMmeasurements and shown in FIG. 8, the approximately size of theresulting carbon nanoparticles prepared hydrothermally using glucose asthe precursor ranges between about 30 nm-50 nm. This is in agreementwith the average particle size obtained from dynamic light scattering(DLS) measurements of 40 nm, as shown in FIG. 9a . Referring now to FIG.9b , the A-NP carbon nanoparticles exhibit a net surface charge of −25mV and fluoresce at a wavelength of 365 nm.

In certain embodiments, to increase the fluorescent intensity of thenanoparticles, the as-prepared carbon nanoparticles were refluxed with asolution of ethanolamine to attach the functional group to the surfaceof the nanoparticles. Referring to FIG. 10, the photoluminescencespectrum shows that the ethanolamine functionalized samples have greaterfluorescent intensity as compared to nanoparticles that lack the surfacetreatment. The top trace is the emission spectrum of the carbonnanoparticles before functionalization, while the lower trace is thespectrum produced by the functionalized carbon nanoparticles. As shownin FIG. 11a , DLS measurements show an average particle size of about 50nm, suggesting that the surface treatment does not lead to any particleagglomeration. Referring to FIG. 11b , the surface charge of the surfacefunctionalized particles is approximately −21 mV.

Coreflood tests were run using a modular Coretest CFS-830 corefloodsystem with high salinity brine having a concentration of about 120,000ppm TDS (total dissolved solids) as a base and displacement fluid. Atthe start of the test, simple dispersion tests of the A-Dots were run atroom temperature using high permeability reservoir rock samples. Thedispersion tests used two consecutive tests of a tracer flood (ortransmission test), followed by a tracer flush (or mobilization test).The tracer flood included the steps of continuously injecting a solutionthat included the tracer particles into one end of the core andmonitoring the effluent from the other end of the core, until theconcentration of the effluent was the same level as the concentration ofthe tracers being injected into the core. The flush test involvedcontinuously injecting a brine solution into the same inlet end of thecore until a tracer concentration of zero was measured at the effluentend of the core.

Tests were conducted using low permeability composite core plugs, whichwere subjected to more stringent and realistic testing conditions interms of reservoir temperature, slug volume and saline concentration.The tests included the use of multiple core plugs in series, lowpermeability plugs (10 millidarcy (mD) or less), high temperature (e.g.,greater than about 95° C.), high pressure (e.g., greater than about 3000psi), and high salinity conditions (e.g., 120,000 ppm or greater). Theprocess included the following steps: first, the core was pre-saturatedwith saturated brine solution (120,000 TDS) and maintained at areservoir temperature (approximately 95° C.) and pressure (approximately3,000 psi) conditions for a few hours. An aqueous solution that includedthe A-Dots (10 ppm) was injected in an amount equivalent to about 0.2core pore volumes (or approximately 20%), and shut-in the setup at theseconditions for a few days. Following the shut-in period, the core wasflushed by injecting a high salinity brine having a TDS of about 120,000at the same end to which the A-Dots were initially introduced. Duringtesting, effluent from the core sample was monitored by collectingapproximately 4-5 mL samples to a fraction collector and conductingfluorescence spectroscopy analyses. Referring to FIG. 12, transportmobility response of the A-Dots following one such test is provided. Acomposite core plug made of two carbonate rock samples having similarpetrophysical properties (including an average brine permeability of9.89 mD, average porosity of 20.3%, and a total pore volume of 18.74 mLfor the two core plugs) was examined. The test employed a 3.8 mL slug ofapproximately 10 ppm (0.001% w/w) A-Dots solution, amounting toapproximately 20% of the total pore volume of the composite core. Thetest used equal injection rates for both the A-Dots solution and theoverflush brine solution at injection rates of about 0.1 mL/min. Thisprocedure created a piston like drive during the slug injection phaseand better emulated the rate at which a water flood front moves into thereservoir during the mobilization (flushing) phase of the test. Theexperiment was shut-in for two (2) days following injection and the slugwas at a maintained temperature of about 95° C. FIG. 12 shows anexcellent A-Dot recovery factor, exceeding about 95%, following theinjection of approximately 4.5 pore volumes of clear brine. This furtheremphasizes the excellent transport mobility of the A-Dots in the Arab-Dmedium.

In a large scale field experiment, approximately 5 kg of A-Dotnanoparticles were disposed in about 255 barrels (bbl) of injectionwater. The solution was mixed on site by recirculating water in the tankat a rate of about 5 bpm (barrels per minute) to provide a 130 ppmsolution of A-Dot tracers. A column full of diesel (120 bbl) was theninjected into the wellbore as a soaker. The well was shut-in for 1 hrand flowed for 1.5 hr to clean the wellbore and remove any accumulatedoil at the top of the formation. The water containing the A-Dots wasinjected at a rate of about 3 bpm and the injection pressure did notexceed 1500 psi. This step was followed with a column full of normalinjection water as an overflush and a column of diesel to serve as adisplacement fluid. The invaded zone was estimated to be about 15 ft.

For the production phase, the well was shut-in for 2 days before it wasflowed to a gas-oil separator and fluid samples were collected at thewellhead in an attempt to monitor and gauge the nanoparticles' stabilityand potential for recovery. After recovery, fluorescence measurementsshowed overall A-Dots recovery on the order of at least about 80%,alternatively at least about 85%, alternatively at least about 90%.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of theinvention. Accordingly, the scope of the present invention should bedetermined by the following claims and their appropriate legalequivalents.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art to which the inventionpertains, except when these references contradict the statements madeherein.

That which is claimed is:
 1. A fluorescent nanoagent for use in a subsurface petroleum reservoir, the nanoagent comprising: a carbon-based nanoparticle core, said nanoparticle core having an average diameter of less than about 100 nm; wherein said nanoparticle core includes a plurality of functional groups appended to the surface thereof; and wherein the fluorescent nanoagent is produced from a solution comprising a sugar, an amino alcohol and deionized water reacted under conditions capable of synthesizing the fluorescent nanoagent.
 2. The fluorescent nanoagent of claim 1, wherein the carbon-based nanoparticle core has an average diameter of less than about 75 nm.
 3. The fluorescent nanoagent of claim 1, wherein the fluorescent nanoagent is detectable at a concentration of about 0.01 ppm.
 4. The fluorescent nanoagent of claim 1, wherein the functional groups appended the surface of the fluorescent nanoagent can be excited at a wavelength between about 400 nm and 500 nm.
 5. The fluorescent nanoagent of claim 1, wherein the functional group appended to the surface of the nanoparticle core is an amino alcohol.
 6. The fluorescent nanoagent of claim 1, wherein the functional group appended to the surface of the nanoparticle core is selected from methanolamine, ethanolamine, and propanolamine.
 7. The fluorescent nanoagent of claim 1, wherein the functional group appended to the surface of the nanoparticle core is ethanolamine.
 8. The fluorescent nanoagent of claim 1, wherein the functional group is appended to the surface of the nanoparticle core is appended with an amide linkage.
 9. The fluorescent nanoagent of claim 1, wherein the sugar is selected from the group consisting of glucose and fructose.
 10. The fluorescent nanoagent of claim 1, wherein the amino alcohol is selected from the group consisting of methanolamine, ethanolamine, and propanolamine.
 11. The fluorescent nanoagent of claim 1, wherein the fluorescent nanoagent is stable at a temperature of about 100° C. to about 200° C.
 12. The fluorescent nanoagent of claim 1, wherein the fluorescent nanoagent is stable at a salinity concentration of about 75,000 ppm to about 120,000 ppm.
 13. The fluorescent nanoagent of claim 1, wherein the plurality of functional groups appended to the surface of the nanoparticle core is ethanolamine, the sugar is glucose, and the amino alcohol is ethanolamine.
 14. A fluorescent nanoagent for use in a subsurface petroleum reservoir, the nanoagent comprising: a carbon-based nanoparticle core, said nanoparticle core having an average diameter of less than about 100 nm; wherein said nanoparticle core includes an ethanolamine functional group appended to the surface thereof; and wherein the fluorescent nanoagent is produced from a solution comprising glucose, ethanolamine and deionized water reacted under conditions capable of synthesizing the fluorescent nanoagent. 